The present invention relates to a continuous process for preparing 3-aminomethyl-3,5,5-trimethylcyclohexylamine by reacting a feed stream comprising 3-cyano-3,5,5-trimethylcyclohexylimine with hydrogen and ammonia over hydrogenation catalysts.
3-Aminomethyl-3,5,5-trimethylcyclohexylamine (isophoronediamine, IPDA) is an important intermediate for polyamides and epoxy resins and for preparing the downstream product isophorone diisocyanate (IPDI) which is used as a component in polyurethanes.
On an industrial scale, 3-aminomethyl-3,5,5-trimethylcyclohexylamine is prepared by reacting 3-cyano-3,5,5-trimethylcyclohexanone (isophorone nitrile, IPN) with ammonia to form 3-cyano-3,5,5-trimethylcyclohexylimine (isophorone nitrile imine, IPNI).
IPNI is subsequently reacted catalytically with hydrogen in the presence of ammonia in a reductive amination reaction to form 3-cyano-3,5,5-trimethylcyclohexylamine (isophoronediamine, IPDA).
The reductive amination can be carried out in a plurality of stages to increase the yield. Thus, EP-A1-0394968 describes a multistage process in which the imino group of IPNI is firstly selectively hydrogenated and the hydrogenation of the nitrile group is subsequently effected under more drastic reaction conditions (higher pressure and temperature). According to the disclosure, the formation of 3-cyano-3,5,5-trimethylcyclohexanol, which is formed by reduction of the 3-cyano-3,5,5-trimethylcyclohexanone which is in equilibrium with IPNI, can be reduced by carrying out the reaction in this way. However, the proportion of further by-products, e.g. cyclic compounds, is from 3 to 7% in the examples.
Good yields are achieved when the reductive amination is carried out in the presence of basic catalysts or compounds. Thus, in DE-A-4010227, the reductive amination is partly carried out in the presence of basic catalysts, and good yields are achieved.
In EP-A1-0623585, it is shown that doping of catalysts with basic components leads to higher yields in the reductive amination.
DE-C-19747913 describes a process for hydrogenating imines and nitrites to form amines, in particular IPDA, with the yield being increased by addition of a quaternary ammonium hydroxide.
In the preparation of IPDA, it is very important not only to achieve a high product yield but to control the isomer ratio between cis-isophoronediamine and trans-isophoronediamine in the reductive amination, since the isomers have different reactivities. Thus, the cis/trans ratio influences the further processing of IPDA and its downstream product IPDI and therefore also the product properties of the products produced from these raw materials. Commercial IPDI generally has a cis/trans ratio (CTR) of from 74:26 to 78:22.
Such an isomer ratio can be set by means of the process described in EP-A1-0394968. The hydrogenation of isophorone nitrile (IPN) is carried out in a temperature range from 10 to 90° C. in a first reaction stage and subsequently at from 90 to 160° C. in the second reaction stage, with the temperature difference between the first and second reaction stages being at least 30° C. and the residence time in the first reaction stage being shorter than in the second reaction stage. Variation of the temperature of the first reaction stage enabled a CTR of from 55.45 to 80:20 to be set, with the yield going through a maximum at a CTR of 76:24.
According to DE-C-19507398, the amount of base used in the hydrogenative amination also has an influence on the isomer ratio. Thus, an increase in the CTR from 60:40 to 68:32 was able to be achieved by decreasing the concentration of base. However, the reduction in the concentration of base also led to a decrease in the yield from 97 to about 92%.
A further increase in the CTR to 75:25 was able to be achieved in DE-A-19756400 when the reductive amination was carried out in the presence of an acid. The yields were in the region of 92%.
The catalyst used in the reductive amination can also influence the isomer ratio. Thus, DE-A-4343890 reports that the CTR is increased by the use of ruthenium rather than cobalt or cobalt-comprising catalysts, but the yield decreases.
The present invention provides a process which makes it possible to increase the isomer ratio at an at least constant or improved yield and to achieve high space-time yields. In particular, the formation of by-products which are difficult to separate off from the reaction mixture should be prevented by means of this process. Furthermore, the process economics should be increased by means of an improved yield.